Heterogeneous networks (HetNets or HTNs) are now being developed wherein cells of smaller size are embedded within the coverage area of larger macro cells and the small cells could even share the same carrier frequency with the umbrella macro cell, primarily to provide increased capacity in targeted areas of data traffic concentration. Such heterogeneous networks try to exploit the spatial variations in user (and traffic) distribution to efficiently increase the overall capacity of the wireless network. Those smaller-sized cells are typically referred to as pico cells or femto cells, and for purposes of the description herein will be collectively referred to as small cells. Such deployments present some specific interference scenarios for which enhanced inter-cell interference coordination (ICIC) techniques would prove beneficial.
In one scenario, the small cells are picocells, which are open to users of the macrocellular network. In order to ensure that such picocells carry a useful share of the total traffic load, user equipments (UEs) may be programmed to associate preferentially with the picocells rather than the macrocells, for example by biasing the SINR threshold at which they will select a picocell to associate with. Under such conditions, UEs near the edge of a picocell's coverage area will suffer strong interference from one or more macrocells. In order to alleviate such interference, some subframes may be configured as “blank” or “almost blank” in the macrocell. A blank subframe contains no transmission from the macrocell. An “almost blank” subframe is a subframe with reduced transmit power (e.g., reduced from a maximum transmit power) and/or a reduced activity subframe (e.g., contains less data than a fully loaded subframe). Legacy UEs (also called terminals) expect to find the reference signals for measurements but are unaware of the configuration of these special subframes. Almost blank subframes may also contain synchronization signals, broadcast control information and/or paging signals.
In order to make use of blank or almost blank subframes (ABSs) effective (note that hereafter the term “special” or “ABS” is used, and should be understood to include both blank and almost blank subframes), signaling is provided from the macrocell to the picocell across the corresponding backhaul interface, known in LTE as the “X2” interface. For LIE Release 10, it has been agreed that this X2 signaling will take the faun of a coordination bitmap to indicate the ABS pattern (for example with each bit corresponding to one subframe in a series of subframes, with the value of the bit indicating whether the subframe is an ABS or not). Such signaling can help the picocell to schedule data transmissions in the picocell appropriately to avoid interference (e.g. by scheduling transmissions to UEs near the edge of the picocell during ABSs), and to signal to the UEs the subframes which should have low macrocellular interference and should therefore be used for RRM/RLM/CQI measurements. (RRM=Radio Resource Management, typically relating to handover; RLM=Radio Link Monitoring, typically relating to detection of serving radio link failure; CQI=Channel Quality Information, derived from the signal strength from the serving cell and the interference from other cells, and typically used for link adaptation and scheduling on the serving radio link).
In Rel-10, the downlink Relative Narrowband Tx Power indicator (DL-RNTP) is defined in TS36.423 in Section 9.2.19. It provides an indication of any DL transmission power restrictions in the cell per resource block in the frequency domain. This information is sent over the X2 interface to a neighbor cell (e.g., base station or eNodeB) so that the neighbor cell may use the information for its own interference aware scheduling. Currently, no methods for cooperatively addressing/handling the ABS and the RNTP information exist.